[PATCH v2] lib/scatterlist: add simple page iterator

[Imre Deak <imre.deak@intel.com>]

Add an iterator to walk through a scatter list a page at a time starting
at a specific page offset. As opposed to the mapping iterator this is
meant to be small, performing well even in simple loops like collecting
all pages on the scatterlist into an array or setting up an iommu table
based on the pages' DMA address.

v2:
- In each iteration sg_pgoffset pointed incorrectly at the next page not
  the current one.

Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 include/linux/scatterlist.h |   50 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 4bd6c06..72578b5 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -231,6 +231,56 @@ size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
  */
 #define SG_MAX_SINGLE_ALLOC		(PAGE_SIZE / sizeof(struct scatterlist))
 
+struct sg_page_iter {
+	struct scatterlist *sg;
+	int sg_pgoffset;
+	struct page *page;
+};
+
+static inline int
+sg_page_cnt(struct scatterlist *sg)
+{
+	BUG_ON(sg->offset || sg->length & ~PAGE_MASK);
+
+	return sg->length >> PAGE_SHIFT;
+}
+
+static inline struct page *
+sg_page_iter_get_page(struct sg_page_iter *iter)
+{
+	while (iter->sg && iter->sg_pgoffset >= sg_page_cnt(iter->sg)) {
+		iter->sg_pgoffset -= sg_page_cnt(iter->sg);
+		iter->sg = sg_next(iter->sg);
+	}
+
+	return iter->sg ? nth_page(sg_page(iter->sg), iter->sg_pgoffset) : NULL;
+}
+
+static inline void
+sg_page_iter_next(struct sg_page_iter *iter)
+{
+	iter->sg_pgoffset++;
+	iter->page = sg_page_iter_get_page(iter);
+}
+
+static inline void
+sg_page_iter_start(struct sg_page_iter *iter, struct scatterlist *sglist,
+		   unsigned long pgoffset)
+{
+	iter->sg = sglist;
+	iter->sg_pgoffset = pgoffset;
+	iter->page = sg_page_iter_get_page(iter);
+}
+
+/*
+ * Simple sg page iterator, starting off at the given page offset. Each entry
+ * on the sglist must start at offset 0 and can contain only full pages.
+ * iter->page will point to the current page, iter->sg_pgoffset to the page
+ * offset within the sg holding that page.
+ */
+#define for_each_sg_page(sglist, iter, pgoffset)				\
+	for (sg_page_iter_start((iter), (sglist), (pgoffset));		\
+	     (iter)->page; sg_page_iter_next(iter))
 
 /*
  * Mapping sg iterator
-- 
1.7.9.5